1. Field of the Invention
The present invention relates to a high performance-permanent magnet used for various electrical appliances, particularly a rapidly cooled magnet of the rare earth-containing alloy system, as well as to a method for producing the same. The present invention provides a magnet exhibiting improved magnetic properties by rapidly cooling an alloy melt of the Fe-R-B system (R is a rare earth element(s) including Y, ditto hereinafter) or Fe-Co-R-B system. The present invention proposes to anneal, under specified conditions, the rapidly cooled and then solidified magnet, to homogenize and stabilize the magnetic properties.
2. Description of the Related Arts
High performance rare earth magnets are Sm-Co magnets which are mass produced by powder metallurgy. Although such Sm-Co magnets can exhibit a maximum energy product of 32 MGOe, they are disadvantageous in that the Sm and Co raw materials are very costly. Such elements as cerium, praseodymium, and neodymium, among the rare earth elements, have a smaller atomic mass than samarium and are inexpensive. In addition, iron is inexpensive. Accordingly, Nd-Fe-B magnets have been recently developed. Regarding these magnets, Japanese Unexamined Patent Publication No. 59-46008 describes sintered magnets, and Japanese Unexamined Patent Publication No. 60-9852 describes rapidly cooled magnets. Although the conventional powder metallurgy process of the Sm and Co powder can be applied to the sintered magnets, they do not utilize the advantage of inexpensive raw materials in one aspect; that is, an alloy ingot of the easily oxidizable Nd-Fe must be finely pulverized to a size of from approximately 2 to 10 .mu.m, and thus the treatment process thereof is difficult to carry out. In another aspect, the powder metallurgy process includes a number of processes, such as melting, casting, rough crushing of ingot, fine crushing, pressing, sintering, and shaping as a magnet.
On the other hand, the rapidly cooled magnet is advantageous in that it is produced by a simplified process wherein the fine pulverizing process is omitted. Nevertheless, an enhancement of the coercive force and energy product, and cost-reductios as well as an improvement in the magnetizing property must be realized to make the rapidly cooled material industrially usable.
In the properties of the rare earth-iron-boron magnet, the coercive force has a temperature sensitive characteristic. The temperature coefficient of coercive force (iHc) is 0.15%/.degree.C. for the rare earth cobalt magnet, but the temperature coefficient of coercive force (iHc) is from 0.6 to 0.7%/.degree.C. for the rare earth-iron-boron magnet; thus being four or more times higher than that of the rare earth cobalt magnet. In the rare earth-iron-boron magnet, there is a serious danger of demagnetization upon a rise in temperature, with the result that the design of a magnetic circuit is limited. In addition, this type of magnet is unusable, for example, for parts in an engine room of automobiles used in tropical regions.
Japanese Unexamined Patent Publication No. 60-9852 claims a composition of 10% or more of rare earth element(s) of Nd or Pr, from 5 to 10% of B, and a balance of Fe, in which composition a high coercive force (iHc) is allegedly provided to the R-Fe-B alloy by a rapid cooling. Heretofore, the outstanding magnetic properties of the R-Fe-B alloy were thought to be attributable to the Nd.sub.2 Fe.sub.14 B compound-phase. Accordingly, with regard to both the sintering method and rapid cooling method, a number of proposals for improving the magnetic properties (Japanese Unexamined Patent Publications Nos. 59-89401, 60-144906, 61-79749, 57-141901, and 61-73861) were based on the experiments of the composition in the vicinity of the above compound, i.e., R=12.about.17%, and B=5.about.8%.
Since the rare earth elements are expensive, preferably the content thereof is low. There is a problem, however, in that the coercive force (iHc) is greatly reduced at a rare earth element content of less than 12%. Note, the coercive force (iHc) is reduced to 6 kOe or less at a rare earth element content of 10% or less, as described in Japanese Unexamined Patent Publication No. 60-9852. In the R-Fe-B alloy, therefore, the coercive force (iHc) is reduced at a rare earth element content of less than 12%. A method of preventing the decrease in the coercive force (iHc) in the composition range as described above, by changing the composition and structure, is not known.
Although the Nd.sub.2 Fe.sub.14 B compound is the basic compound used in both the sintering method and the rapid cooling method, the magnets produced by these methods not only are different in the production methods but also belong to fundamentally different types of magnets, in view of the alloy structure and coercive force-generating mechanism, as described in Oyobuturi (Applied Physics) No. 55, Vol. 2 (1986), page 121. Namely, in the sintered magnet, the grain size is approximately 10 .mu.m. A magnet with such a grain size is, compared with the conventional Sm-Co magnet, a nucleation type, in which the nucleation of inverse magnetic domains determine the coercive force. On the other hand, the rapidly cooled magnet is a pinning type, in which the coercive force is determined by the pinning of the magnetic domain-wallls due to the extremely fine structure of fine particles from 0.01 to 1 .mu.m in size surrounded by amorphous phases. Accordingly, any approach to an improvement the properties of the magnets must first fully consider the differences in the generation mechanisms of the coercive force.